1. Field of the Invention
The present invention relates to braided metallic desoldering wicks for removing solder from a soldered electrical connection. The invention is more particularly directed to a desoldering wick having a flux coating containing a dye for visually demarking the boundary between used and unused portions of the wick.
2. Description of the Prior Art
For several years braided metallic desoldering wicks have been used to remove solder from soldered electrical connections. For example, in order to remove an electrical component from a printed circuit board for replacement it is necessary to melt the solder which electrically and mechanically connects the component to the circuit board and to remove the molten solder. This is done by applying heat to the connection as with a soldering iron or gun and contacting the molten solder globule with the tip of the braided desoldering wick. The molten solder then flows up the wick and away from the connection through capillary action. In order to remove metallic oxides and other contaminants from the connection to be desoldered the braid is typically coated with a flux agent such as for example rosin flux.
An effective flux-coated solder-wicking braid should have the following properties:
1. Upon pressing the braid with a hot soldering iron onto the solder to be wicked the solder should quickly fuse, wet the metal of the braid and flow into it. PA1 2. When the braid bearing the solder is removed from the work there should be no fillets of solder left on the work. PA1 3. Some of the flux used on the braid will flow onto the work leaving a residue. This residue should be only a light tan color and not dark tan or brown. PA1 4. The residue on the work should be electrically non-conducting and non-corrosive. PA1 5. To effectively use fresh portions of braid for subsequent solder removal there should be a clear indication of the boundary between braid containing old solder and unused braid.
A form of solder-wicking braid which has been used employs braided copper wire coated with rosin flux. If the copper is clean then the braid functions well according to the above criteria. If the copper is contaminated with oxide either through inadequate cleaning of the braid before flux coating or through aging of the flux coated braid, then in operation the molten solder wets the copper sluggishly since the contamination must first be removed chemically by action of the flux. Further, the flux with its burden of copper now leaves a brown residue on the work because of the chemical compound formed between the copper contamination and the rosin flux.
The problem associated with the use of copper wicks have been avoided by applying a tin coating to the copper strands of the braid, the tin being applied by immersing the copper wires, before braiding, in molten tin. Such a tin coated copper braid is disclosed in U.S. Pat. No. 3,715,797 issued to Jackson et al.
Tin resists formation of contamination upon aging better than copper and is more readily wet by molten solder since tin is a component of solder. Furthermore the reaction of tin oxide contaminants with rosin does not give products which color the rosin flux residue. However, since tin and solder are both silvery colored metals the boundary between used and unused braid is not as clear as it is with copper braid. The Jackson patent referred to above suggests that the demarcation problem in tin coated braid may be solved by the expedient of dying or otherwise coloring the tin coating such that the solder captured will cover up the dye thereby showing by contrast the used portions of the braid material. In practice however application of this approach is difficult because of the fact that all methods of coloring metals involve formation of a surface film which inhibits wetting by molten solder. For example, aluminum surfaces may be colored by anodizing to form a porous oxide surface which is then dye-receptive. However, the surface is thereby made highly resistant to wetting by molten metals. The surface of tin has a thin coating of tin oxide resulting from exposure to the atmosphere. This coating if not too thick is not a significant barrier to wetting by molten solder. The coating will either be removed by chemical reaction with the rosin flux or will break up when the tin is melted during the soldering. It is essential to remove or undermine the tin oxide coating to achieve good wetting by molten solder. No colored substance has been found except a colored metal such as gold or copper which can be used to color a tin surface and be covered up by wicked solder.